1. Field of the Invention
The present invention relates to a technique of measuring the surface position of a substrate by irradiating the surface of the substrate with light and photo-receiving the light reflected by the surface of the substrate and, more particularly, to a scanning exposure apparatus comprising an apparatus which measures the surface position of a wafer serving as an exposure target substrate.
2. Description of the Related Art
The manufacture of a fine semiconductor element by photolithography, such as a semiconductor memory or a logic circuit, a liquid crystal display element, or a thin-film magnetic head, adopts a projection exposure apparatus which causes a projection optical system to project and transfer the circuit pattern drawn on the reticle (mask) onto a wafer, or the like.
Along with an increase in the degree of integration of semiconductor elements, a demand has arisen that a projection exposure apparatus should project and transfer the circuit pattern of the reticle onto a wafer by exposure with a higher resolving power. A minimum dimension (resolution) with which the projection exposure apparatus can transfer is proportional to the wavelength of exposure light, and inversely proportional to the numerical aperture (NA) of a projection optical system. The shorter the wavelength is, the higher the resolution becomes. In recent times, the light source is shifting from superhigh pressure mercury lamps, a g-line with a wavelength of approximately 436 nm, and an i-line with a wavelength of approximately 365 nm, to a short-wavelength KrF excimer laser, with a wavelength of approximately 248 nm, and an ArF excimer laser, with a wavelength of approximately 193 nm. A practical application of an F2 laser, with a wavelength of approximately 157 nm, is also in progress. A demand has also arisen for a further increase in exposure area.
To meet these demands, a step and scan exposure apparatus, i.e., a scanner, which scans a reticle and a wafer relative to each other at a high velocity by using a rectangular slit-like exposure area to accurately expose a wide area, is becoming mainstream in place of a step and repeat exposure apparatus, i.e., a stepper, which reduces an almost square-shaped exposure area to execute cell projection for a wafer.
As shown in FIG. 1, the scanner comprises an illumination unit 10, a reticle stage 25, which supports a reticle 20, a projection optical system 30, a wafer stage 45, which supports a wafer 40, a focus/tilt detection system 50, an alignment detection system 70, and a control system 60. The control system 60 comprises a CPU and a memory, electrically connects to the illumination unit 10, the reticle stage 25, the wafer stage 45, the focus/tilt detection system 50, and the alignment detection system 70, and systematically controls the overall operation of the apparatus.
The alignment detection system 70 detects a misalignment of the wafer 40 in the X- and Y-axis directions. Referring to FIG. 1, the alignment detection system 70 is a so-called off-axis optical system, which uses non-exposure light and is inserted on an optical axis that is shifted from the optical axis of the projection optical system 30.
The wafer stage 45 supports the wafer 40 via a wafer chuck 46. At least three wafer chuck marks are laid out on the wafer chuck to cause the focus/tilt detection system 50 to acquire Z height information and to cause the alignment detection system 70 to acquire X-Y position information. Using a linear motor, or the like, the wafer stage 45 moves the wafer 40 and wafer chuck 46 in the X-axis direction, Y-axis direction, Z-axis direction, and the rotation directions about the respective axes. A laser interferometer, for example, monitors the positions of the reticle stage 25 and the wafer stage 45, to drive the respective stages at a constant velocity ratio.
The focus/tilt detection system 50 detects the position information about the surface position, in the Z-axis direction, and the surface tilt of the wafer 40 during exposure.
Before a predetermined position on the wafer 40 reaches the exposure slit area during exposure, the scanner causes the focus/tilt detection system 50 to measure the surface position at the predetermined position, and executes a correction to match the wafer surface with an optimal image forming position, in exposing the predetermined position.
Various kinds of focus and tilt measurement methods are proposed; see, e.g., Japanese Patent Laid-Open No. 1994-260391.
In recent years, along with a recent decrease in the wavelength of exposure light and a recent increase in the NA of a projection optical system, the depth of focus is becoming very small. A so-called focus accuracy for matching the wafer surface to be exposed with an optimal image forming position is also becoming stricter. Nowadays, especially, a measurement error of the surface position due to the density fluctuations of the pattern on the wafer or the thickness non-uniformity of the resist applied to the wafer is becoming non-negligible.
The measurement error due to the thickness non-uniformity of the resist occurs when a step that is small for the depth of focus, but is fatal for focus measurement is formed near the peripheral circuit pattern or scribe line. Since the tilt angle of the resist-coated surface increases, the focus/tilt detection system 50 detects reflected light at an angle deviated from the regular reflection angle upon reflection or refraction.
The measurement error due to the density fluctuations of the pattern on the wafer occurs when, e.g., the reflectance of the wafer varies such that a sparse pattern area on the wafer has a high reflectance, while a dense pattern area on the wafer has a low reflectance, as shown in FIG. 10. The reflection intensity of the reflected light detected by the focus/tilt detection system 50 is changed as a result, causing an asymmetry, as indicated by (B), with respect to a signal waveform (A) that is free from any pattern density fluctuations. A measurement error, or offset, results, upon a signal process such as a barycentric process.
The foregoing measurement errors degrade the performance of the CD. That is, as shown in FIG. 11, in generating an approximate plane within a certain area in the wafer to match the exposure image forming plane with the wafer surface, when the measurement point mk3 suffers from a large measurement error due to the reflectance difference by Cu, a deviation ΔZ from an actual plane occurs as defocus. When the measurement error is stably measured per unit area (shot) in the wafer, it is possible to manage it as an offset. However, a place like the measurement point mk3 in FIG. 11 exhibits a variation in measurement value. In many cases, the place exhibits low reliability as a measurement error, and even as an offset.